It is becoming increasingly important in health care programs to know the concentrations of biologically essential elements in normal subjects and to be able to accurately measure the smallest deviation that is significant with respect to disease. This is particularly true for elements known to be toxic. There have been and still are many problems associated with trace analyses. Chief among these problems are (a) the inability to accurately conduct multielement trace analyses, i.e., at the nanogram level, (b) the non-existence of systems and techniques which allow for rapid and economical trace metal analyses, (c) the inability to handle small sample sizes, (d) sample losses and contamination, and (e) spectral background, where spectrochemical techniques are employed. Many of the problems have been overcome by the use of inductively-coupled plasma techniques. See, for example, Knisely et al., Clinical Chemistry, Vol. 19, 8, pages 807-812 (1973) and Fassel et al., Analytical Chemistry, Vol. 46, 13, pages 1110A-1120A (1974). However, the problem of background interference remains from such sources of stray light as continuum plasma emission, molecular oxide band emission, near and far scatter, Rowland ghosts, etc., G. F. Larson et al., Applied Spectroscopy, 30, 384 (1976).
Several approaches to background correction have been used. Among the approaches employed have been (a) measuring the background emitted by a blank sample at each analytical line and using that value for correction, (b) movement of an alignment refractor plate behind the entrance slit of an emission spectrometer to permit the measurement of background just off each analytical wavelength with actual samples, (c) setting aside a specific readout channel on a spectrometer for background measurement at a selected wavelength, and (d) the use of photoelectric techniques to measure the background and the analytical line plus background simultaneously.
Among the earlier solutions to background correction, there have been proposed various optical scanning techniques, such as movement of the entrance or exit slits, or of the diffraction grating, or indeed movement of some means within the light beam itself, such as the above-referenced alignment refractor plate. Such prior art is represented for example by Snelleman et al., "Flame Emission Spectrometry with Repetitive Optical Scanning in the Derivative Mode," Analytical Chemistry, Vol. 42, No. 3, March 1970, pages 394-398. Here a refractor plate located near an entrance or exit slit is sinusoidally operated. A lock-in-amplifier is used to extract the wanted line emission signal from the unwanted varying spectral background. When the wavelength scan is sinusoidal and centered on the emission line of interest, only about 10 to 20% of the scan time is devoted to signal measurement. This type of wavelength modulation wastes desired signal integration time. Another reference teaching similar techniques is Visser et al., "A Device for Repetitive Scanning of a Spectral Line," Applied Spectroscopy, Vol. 30, No. 1, 1976, pages 72-73. In general it may be said of such prior art arrangements that the same provides sinusoidal type optical scans, which have the inherent disadvantage of providing the least amount of time for observation (i.e., the maximum rate of scan) at the point of maximum interest, i.e., at and proximate the wavelength associated to a spectral line of interest for a particular trace substance. Consequently, with a sinusoidal type scan only 10 to 20% of the total scan time is devoted to the spectrum portion of interest, whereas the remainder is concentrated in observing the background on either side of the wavelength of interest and relatively remote therefrom in terms of the necessity for accurate background correction relating to analysis of complex substances.
Another solution to the problem of background interference is that proposed by Gordon et al., Applications of Newer Techniques of Analysis, pages 39-42 (Plenum, New York, 1973). The Gordon et al. approach utilizes a two-position rotary refractor plate which is at all times in the path of the light beam. By stepping the refractor plate between the two positions, the emission line and the background on one side of the emission line respectively can alternately be measured. This approach is disadvantageous in that inter alia background information is obtained with regard to only one side of a spectral line and the "scan" is discontinuous, thus not presenting the true intensity situation at and proximate to the wavelength of interest. Another similar solution is proposed by Skogerboe et al., Applied Spectroscopy, Vol. 30, No. 5 (1976), pages 495-500. This approach utilizes an optical scan refractor plate mounted on a tuning fork such that the refractor plate can be moved in and out of the optical path in a "squarewave" mode. A principal disadvantage of this approach, again, is that because of the in and out movement of the refractor plate, only one portion of the background is observed, i.e., only one side of the spectral line is examinable. It is also necessary to correct for light transmission losses due to reflection and scattering when the refractor plate is in the optical path.